Microprocessor chips generally include a logic unit and cache memory. If both the logic unit and the memory devices of a microprocessor are arranged in a two-dimensional (2-D) pattern, limitations on the physical size of the chip (imposed by poor process yields for large-area chips) may lead to restrictions on the amount of cache memory. The performance of the microprocessor may therefore be severely limited.
To address the problem of providing adequate cache memory for microprocessors (and more generally the problem of 2-D real estate on a chip), a number of researchers are exploring methods for building three-dimensional (3-D) integrated circuits. A typical 3-D fabrication process includes building devices on wafers which are then thinned to less than 20 μm; providing vertical interconnections through the wafers; stacking the wafers so that vertical connections are established between wafers at different levels; and bonding the wafers with a suitable material. See, for example, J.-Q. Lu et al., “Fabrication of via-chain test structures for 3D IC technology using dielectric glue bonding on 200 mm wafers,” Materials Research Society ULSI XVII Conference Proceedings 151 (2002); P. Ramm et al., “Interchip via technology by using copper for vertical system integration,” Materials Research Society Advanced Metallization Conference 159 (2002); and Rahman et al., “Thermal analysis of three-dimensional integrated circuits,” IEEE International Interconnect Technology Conference Proceedings 157 (2001). Significant problems in the present state of the art of 3-D integration include (1) the need for reliable wafer bonding; (2) stringent wafer cleanliness and flatness requirements; (3) the need for reliable, low-resistance inter-wafer vertical connections; (4) stringent wafer-to-wafer lateral registration requirements; and (5) the need for efficient heat conduction through the 3-D device.
A process for making 2-D chip-to-chip interconnects is described in “Process for making fine pitch connections between devices and structure made by the process,” U.S. Pat. No. 6,444,560 assigned to International Business Machines Corporation, the disclosure of which is incorporated herein by reference. As noted in this patent, chips having different functions and possibly of different materials may be connected through a wiring layer of polyimide using stud/via connections between the wiring layer and the respective chips. It is desirable to extend the techniques discussed in this patent to achieve 3-D chip-level and wafer-level integration.